One summer day in 2001, John Styrsky was searching the rainforest in Panama for a bird’s nest. As he wandered around, he accidentally bumped into a tree called an acacia. Suddenly, he recalls, “I started feeling these really intense stinging sensations on my neck and back.”

Panicked, the biologist took off his shirt and tried to brush off whatever was causing the pain. He discovered about 15 ants from the acacia tree swarming his skin. Known as acacia ants,these insects grab their victim’s sk...